Large bowel adenomatous polyps present a unique opportunity to conduct an intervention trial because of the high prevalence rate in the general population, the high polyp recurrence rate in those who have undergone polypectomy, and the link between polyps and cancer. It is generally accepted that large bowel adenomas are a requisite precursor lesion for most large bowel cancers. Given the strong evidence for the polyp- cancer sequence, an intervention that reduces the recurrence of large bowel polyps would have a strong likelihood of reducing the incidence of large bowel cancer. The major objective of this study is to determine whether a low fat, high fiber, high fruit and vegetable dietary pattern will decrease the recurrenc rate of large bowel adenomatous polyps. This is a multi-center randomized controlled trial involving 2,000 men and women. Study participants are being randomized into either the experimental diet group or a control group (usual diet). Recruitment will take up to two years, and the followup time from randomization is four years. The study has three secondary objectives: 1) to investigate the relation between the dietary intervention and several putative intermediate endpoints in large bowel carcinogenesis, particularly markers of colonic epithelial cell proliferation; 2) to evaluate whether these intermediate endpoints correlate with subsequent neoplasia (adenoma formation); and 3) to determine the extent to which changes in the intermediate endpoints account for the observed effect.